ABSTRACT: The vast majority of the roughly 100,000 cochlear-implant (CI) users have severe-to-profound hear- ing loss, but only receive a single CI in one ear. Yet, not having access to sound in two ears severely hampers the auditory systems' ability to overcome background noise, which has many consequences including increasing the amount of effort it takes to communicate. Even with access to sound in both ears, two-ear or ?binaural? benefits for bilateral CI (BI-CI) and single-sided deaf CI (SSD-CI) users are small compared to normal-hearing listeners. Because of possible frequency mismatch between the two ears, it is unclear if the current standard approach of programming the CIs maximizes speech understanding for two ears, particularly in environments with multiple sound sources. Our long-term goal is to understand and maximize speech understanding and bin- aural benefits imparted by CIs by improving interaural frequency, level, and timing encoding. The next steps toward this goal and the objectives here are to evaluate interaural frequency mismatch and to develop novel CI programs (MAPs) to correct for mismatch. Our central hypothesis is that binaural function in BI-CI and SSD-CI users is suboptimal because of interaural frequency mismatch. Our approach will be to optimize the frequency- allocation in the poorer ear to minimize the mismatch and potentially improve speech understanding and spatial hearing. With strong preliminary data in hand, the central hypothesis will be tested by pursuing three specific aims: (1) Assess interaural frequency mismatch for individual electrodes in BI-CI and SSD-CI users. (2) Compare the acute functional benefits and limitations for the standard MAP and novel MAPs that minimize interaural fre- quency mismatch in BI-CI and SSD-CI users. (3) In a longitudinal study, evaluate the extent to which standard MAP and novel MAPs cause changes in pitch processing, binaural processing, and speech understanding in BI- CI and SSD-CI users. The expected outcome is improved binaural function for BI-CI and SSD-CI users, without sacrificing speech understanding. This research is significant because it will produce a positive impact in CI users' quality of life by improving communication in quiet and noisy environments. This proposal is innovative because it uses knowledge of binaural neural processing to provide a novel, non-invasive, and inexpensive method to reprogram CIs for bilateral hearing.